1. Field of the Invention
The present invention relates in general to an apparatus for winding up automatically a seat belt used in a vehicle to protect occupant and in particular to a mechanism for controlling the force for winding up or retracting the seat belt.
2. Description of the Prior Art
In hitherto known seat belt winding or retracting apparatus, a spiral spring is employed which has one end connected to a belt winding shaft and the other end fixedly connected to a stationary part of the winding apparatus through an accommodating box or case in which the seat belt is stored. When the seat belt is pulled outwardly from the apparatus by occupant in the vehicle to be bound around the body of occupant for protection, the spring is increasingly tensioned to store spring energy as the belt is paid out. Such stored energy is utilized for automatically winding up or retracting the seat belt into the apparatus when the occupant puts off the binding belt. The hitherto known type seat belt winding apparatus as described above has been disadvantageous in that excessively great binding force is applied to the occupant who thus has uncomfortable feeling and is restricted considerably in his movement, since the spring force applied to the seat belt is increased as the belt is pulled out to be put on the occupant. This is particularly true in the case where the lap belt is formed integrally with a shoulder belt and the winding apparatus is installed on the side of the shoulder belt. Such disadvantage may be eliminated by employing a spring having a correspondingly reduced spring force. However, the spring force for winding up the seat belt for storage will then become insufficient to retract the belt smoothly and rapidly.